1. Field of the Invention
The invention relates to a manufacturing method of a hydrogen-doped silicon single crystal, in which a silicon single crystal is grown by the CZ method under an inert atmosphere containing hydrogen.
2. Description of Related Art
One of the typical manufacturing methods of a silicon single crystal for a base material of a silicon wafer is a rotation pulling method referred to as the CZ method (Czochralski method). The manufacture of a silicon single crystal by the CZ method is performed with known steps including dipping a seed crystal in silicon melt formed in a quartz crucible, and then pulling the seed crystal up while the crucible and seed crystal are being rotated, to grow a silicon single crystal beneath the seed crystal.
For the atmosphere for the CZ pulling, inert gases (mainly Ar gas) have been traditionally applied to suppress various chemical reactions between silicon melt, furnace member and a crystal, and to avoid contamination of impurities produced as by-product. Furthermore, avoidance of metal contamination by utilizing gas flow in the furnace generated by an abundant gas supply improves a quality of the pulled-up crystal.
With regard to the furnace atmosphere, the effect of mixing minute amounts of hydrogen gas therein has been currently reported (for example, Japanese Unexamined Patent Application, First Publication No. S61-178495, Japanese Unexamined Patent Application, First Publication No. H11-189495, Japanese Unexamined Patent Application, First Publication No. 2000-281491, and Japanese Unexamined Patent Application, First Publication No. 2001-335396). According to his technology, hydrogen can reduce or eliminate vacancy defects by acting on Grown-in defects, especially on vacancy defects represented by COP, induced in a crystal, to the same extent as similar to nitrogen doping for silicon melt.
In such hydrogen doping technologies in the CZ pulling, a mixture of hydrogen gas and inert gas is introduced into the furnace instead of the inert gas alone. The mixed gas is introduced from a top of a pull chamber and exhausted from a bottom of a main chamber to the outside of the furnace, to replace the entire interior of the furnace with a predetermined atmosphere, as performed with inert gas in the conventional CZ pulling. In other words, the mixed gas passes through from an uppermost part to a lowermost part in the CZ pulling furnace.
It has, however, been found that a mixed gas introduction mode cannot allow the hydrogen gas in the mixed gas to effectively inhibit the formation of vacancy defects. This is reasoned as follows.
The primary object of hydrogen-gas doping is to inhibit aggregation of vacancies in silicon by couple hydrogen with the vacancies before the vacancies causing COP aggregate themselves. For this purpose, it is necessary to supply hydrogen gas to silicon before the silicon is cooled down to a temperature of vacancy aggregation (around solidification temperature of a melt). The hydrogen gas introduced into a furnace is supplied to the external surface of a pulled-up crystal, but hardly influences the crystal quality. This is because, in considering the times for pulling up and the diffusion distance of a hydrogen atom in silicon, the hydrogen entered into the crystal from its external surface does not reach the core of the crystal, thereby it does not exert an effect of inhibiting COP formed in the crystal cores. Therefore, to effectively contribute to quality improvement, it is advantageous that the crystal be grown from a silicon melt containing hydrogen after a pre-determined amount of hydrogen gas has been supplied to a surface of the melt in a crucible. For such a purpose, it is advantageous that the hydrogen gas be directly supplied to a location adjacent to a solid-liquid interface or in a silicon melt.
However, the crucible storing the silicon melt is placed in a main chamber at a lower part of the CZ pulling furnace significantly distant from the mixed gas inlet. The hydrogen gas in the mixed gas is easily adsorbed by an internal surface of the pulling furnace or various members in the furnace, especially by carbon members. Furthermore, an allowable amount of the hydrogen gas is restricted to minute amount, at most 3% by volume, in view of preventing an explosion. Therefore the hydrogen gas introduced into the furnace seldom reaches the surface of a melt in the crucible, and the generation of COP is not inhibited effectively.
The object of the invention is to provide a manufacturing method of a hydrogen-doped silicon single crystal, wherein the method can produce a high quality hydrogen-doped silicon single crystal by the CZ method without increasing the concentration of hydrogen gas in a mixed gas.
To achieve the object, the inventors have extensively studied ways to supply hydrogen gas, wherein a silicon single crystal is grown by the CZ method under an inert atmosphere containing hydrogen; and therefore, have reached the following conclusions.
As stated above, to allow hydrogen gas to effectively act on COP inhibition, some amount of hydrogen gas must be supplied to a surface of a melt in a crucible, especially to a part adjacent to a solid-liquid interface of the melt surface. The major factor restricting such supply is that, since a minute amount of hydrogen gas contained in a mixed gas is introduced into an uppermost part of the chamber mostly distant from a crucible storing a silicon melt, the hydrogen gas travels through a long passage to reach the silicon melt and is adsorbed by an internal surface of the CZ pulling furnace or various members in the furnace, especially by carbon members, on the way through such a passage.
Therefore, to allow hydrogen gas to effectively act on COP inhibition, it is necessary to shorten the passage in a furnace for the hydrogen gas to travel through after being injected into the furnace until reaching the silicon melt.
The present invention has been accomplished based on this view point.